if you set outlet gauge pressure to 0 pa, the pressure in all cells should be positive unless for supersonic flows or when your case doesn't converge yet. check whether the pressure at the outlet is zero or not and also check residuals.

your gauge pressure in those zones is negative not absolute one. (e.g. if you get -1000 pa, it means that your absolute pressure there is 100325 pa) it just shows that pressure there is lower that the value at the outlet.